There's a couple of issues you've brought up that suggest your in law's do not respect you. It's not just the illicit baths - which are wrong, they know it's wrong and they do it anyway - it's also the snide comments about your parenting skills. It's your choice how you raise your child. OK, they may have more experience, but that's about offering advice if it's required, not undermining what you're doing.

How long do you have to live with for? Are there other options until the house is built?

They need to get the message loud & clear that you have rules as to how you're raising your child, who has what contact with them and if they don't like it, you'll have to find other arrangements.

And I agree with pp - wait til after the wedding. Then have a sit down chat with them. Make sure your partner is in total agreement with you, or it could create a war. He needs to put you before them.

GL